Heat the chicken stock in a saucepan until it is simmering. Add the peas and simmer for 2 minutes, or until just tender. Stir in the tarragon and salt.
Transfer the mixture to a food processor and blend for a few short blasts to a rough-textured soup. Return the soup to the saucepan and keep warm over a low heat.
Using a pair of tongs, heat each slice of pancetta over the gas flame of your hob for a few seconds until it crisps up. (Alternatively, preheat your grill to its highest setting, lay the slices of pancetta onto a baking tray and grill for 1-2 minutes, or until crisp.)
To serve, ladle the soup into bowls. Break the crisp pancetta into pieces and sprinkle on top of the soup.